This invention relates to polymer electrolyte membrane (PEM) fuel cells, more specifically it relates to the membrane electrode assembly (MEA). The multilayer assembly of the membrane sandwich that is between the two electrodes is commonly called the MEA. The MEA is sandwiched between the collector/separator plates. Collector plates collect and conduct electrical current. Separator plates separate the gases in adjacent cells in multicell configurations. Separator plates are also called bipolar plates because they physically and electrically connect the cathode of one cell to the anode of the adjacent cell. They provide the pathways for flow of reactant gases and provide the structural rigidity of the cell.
Requirements of a fuel cell membrane include high proton conductivity, the ability to provide an adequate barrier to mixing of fuel and reactant gases, and chemical and mechanical stability. Membranes are often perfluorocarbon-sulfonic acid ionomers, which are co-polymers comprising tetrafluoroethylene and perfluorosulfonate monomers. Nafion is the best known of this family of materials and is a perfluoro-sulfonylfluoride ethyl-propyl-vinyl ether.
A fuel cell electrode comprises a thin catalyst layer between the ionomer membrane and a porous, electrically conductive substrate. The electrochemical reactions of the fuel cell occur at the catalyst surface. The most common catalyst in PEM fuels cells is Pt, which catalyzes both oxygen reduction and hydrogen oxidation. In early fuel cells, large amounts of Pt were used (up to 28 mg/cm2). In the late 1990s, the use of supported catalysts reduced this to 0.3 to 0.4 mg/cm2. It is desirable to minimize the amount of Pt while maximizing the Pt surface area by using small particles finely dispersed onto the surface of the catalyst support, which is typically a carbon powder (about 40 nm size) with a high mesoporous area (>75 m2/g). One key to improving PEM fuel cell performance is increasing the active Pt surface area.
There are two common ways to prepare the catalyst layer and attach it to the ionomer membrane, thereby forming the MEA. One can deposit the catalyst layer on the porous substrate (the gas diffusion layer), which is typically carbon fiber paper or cloth, then hot-press it to the membrane. Alternatively, one can apply the catalyst directly to the membrane. Current methods of applying catalyst include spreading, spraying, sputtering, painting, screen printing, electrodeposition, evaporative deposition, and impregnation reduction.
An important design consideration is that the catalyst should be simultaneously physically touching all three phases: the membrane (for proton transport), the porous carbon (for electron transport) and the gas (for oxygen transfer) in order to catalyze the oxygen reduction reaction at the cathode. Current construction techniques to provide the desired connections are frequently dependent on methods such as brush-painting or spraying a mixture of carbon-supported Pt and solubilized membrane polymer onto the solid membrane. This approach causes a significant portion of the Pt to lack good contact with the membrane, preventing access to protons, or to be buried in the polymer or carbon, preventing access to oxygen.
Some Pt alloys that are very stable (PtCr, PtZr, and PtTi) can be used in PEMs, but dissolution of the base metal by the perfluorinated sulfonic acid in the electrocatalyst layer and membrane precludes the use of most other catalysts in many fuel cell applications.